A Dual-Wavelength Pulsed Laser Processing Platform for a-Si Thin Film Crystallization

Abstract

Interest in laser crystallization (LC) of silicon (Si) thin films has been on the rise in fabrication of polycrystalline silicon (pc-Si) based thin/ultrathin photovoltaic solar cells and Si based thin film transistors (TFT). Laser based fabrication of device quality pc-Si thin films at room temperature is expected to be a key enabling technology because of its low energy, material and process time budget. Fabrication of high-quality pc-Si thin films without pre-/post-treatment at large is a disruptive technology which has the potential to revolutionize the Si thin film industry. We hereby describe in detail a multi-wavelength laser processing platform specially developed for crystallization of amorphous silicon (a-Si) thin films into pc-Si thin films. The platform has three main stages. The first stage consists of a nanosecond pulsed ytterbium (Yt3+) doped fibre-laser with a master oscillator power amplifier architecture, operating at a wavelength of 1064 nm with an adjustable repetition rate between 80 kHz–300 kHz. The output beam has a maximum power of 18 W with a pulse energy of 90 µJ. The pulse durations can be set to values between 15 ns–40 ns. The second stage has free-space optical elements for second harmonic generation (SHG) which produces an emission at a wavelength of 532 nm. Conversion efficiency of the SHG is 25% with an output pulse energy of 20 µJ. The platform provides two wavelengths at either 1064 nm or 532 nm in crystallization of a-Si films for different crystallization regimes. The last stage of the platform has a sample processing assembly with a line-focus, which has an x-y motorized stage on a vibration isolated table. Speed of the motorized stage can be set between 1 mm/s–100 mm/s. Stage speed and repetition rate adjustments help to adjust overlap of successive pulses between 97.22–99.99%. Our platform has variety of tune parameters that make it a uniquely flexible system for delicate Si thin film crystallization. A large selection of operational parameter combinations, the wavelength selection and simultaneous x-y scanning capability allow users to crystallize Si films on various substrates optimally. The operation wavelength choice can be done by considering optical absorption and thickness of a-Si films on different types of substrates. Hence, delivering precise amount of absorbed energy in the line-focus irradiation is useful in increasing the average size of crystalline domains; moreover, nucleation of crystallites can be initiated either from the top or bottom interface of the film. Continuous and simultaneous motion of the stage in two dimensions allows to process arbitrary continuous pc-Si geometries in a-Si film. In summary, our multi-wavelength laser processing platform offers all-in-one LC utility for intricate LC-Si processing.